Conventionally, in optical communication systems employing a single-mode optical fiber as their transmission line, light in a 1.3-.mu.m wavelength band or 1.55-.mu.m wavelength band has often been utilized as signal light for communications. Recently, from the viewpoint of lowering the transmission loss in the transmission line, light in the 1.55-.mu.m wavelength band has been increasingly used. Such a single-mode optical fiber employed in a transmission line for the light in the 1.55-.mu.m wavelength band (hereinafter referred to as optical fiber for 1.55 .mu.m) has been designed such that its wavelength dispersion (phenomenon in which pulse waves spread due to the fact that the propagating speed of light varies depending on wavelength) of the light in the 1.55-.mu.m wavelength band becomes zero (yielding a dispersion-shifted fiber with a zero-dispersion wavelength of 1550 nm). As such a dispersion-shifted fiber, for example, Japanese Patent Application Laid-Open No. 62-52508 proposes a dispersion-shifted fiber having a dual shape core structure in which its core region is constituted by an inner core and an outer core having a refractive index lower than that of the inner core. Also, Japanese Patent Application Laid-Open No. 63-43107 and No. 2-141704 each propose a dispersion-shifted fiber having a depressed cladding/dual shape core structure in which its cladding region is constituted by an inner cladding and an outer cladding having a refractive index greater than that of the inner cladding. Further, Japanese Patent Application Laid-Open No. 8-304655 and No. 9-33744 each propose a dispersion-shifted fiber having a ring core structure.
On the other hand, in recent years, the advent of wavelength-division multiplexing transmission and optical amplifiers has enabled the realization of long-haul transmission. Hence, in order to avoid nonlinear optical effects, there has also been proposed a dispersion-shifted fiber employing the above-mentioned dual shape core structure, depressed cladding/dual shape core structure, or the like, with the zero-dispersion wavelength shifted to a wavelength shorter or longer than the center wavelength of the signal light (Japanese Patent Application Laid-Open No. 7-168046 and U.S. Pat. No. 5,483,612). Here, the nonlinear optical effects refer to phenomena in which, due to nonlinear phenomena such as four-wave mixing (FWM), self-phasemodulation (SPM), cross-phasemodulation (XPM), and the like, signal light pulses are deformed in proportion to the density in light intensity or the like. These effects become factors for restricting transmission speed or the repeater spacing in a relaying transmission system.
In the above-mentioned dispersion-shifted fibers proposed for wavelength-division multiplexing transmission, their zero-dispersion wavelength is set to a value different from the center wavelength of the signal light, thereby restraining the nonlinear optical effects from occurring, or their effective area A.sub.eff is elongated so as to reduce the density in light intensity, thereby restraining the nonlinear optical effects from occurring.
In particular, in the dispersion-shifted fiber shown in the above-mentioned Japanese Patent Application Laid-Open No. 8-304655 or No. 9-33744 employing a ring core structure, the dispersion slope is made smaller, whereas the effective area A.sub.eff is made greater, thus realizing a fiber characteristic suitable for wavelength-division multiplexing transmission.
Here, the effective area A.sub.eff is, as disclosed in Japanese Patent Application Laid-Open No. 8-248251, given by the following expression (1):
A.sub.eff =2.pi.(.intg..sub.0.sup..infin. E.sup.2 rdr).sup.2 /(.intg..sub.0.sup..infin. E.sup.4 rdr) (1)
wherein E is the electric field accompanying the propagated light, and r is the radial distance from a core center.
On the other hand, the dispersion slope is defined by the gradient of the graph indicating the dispersion characteristic in a predetermined wavelength band.